1. Field of the invention
The present invention relates to a method of testing telephone line equipment, more particularly equipment that includes an array of resistors, in particular a hybrid array of resistors, for effecting diverse functions such as powering the telephone line and limiting the ringing current. It also relates to a line equipment adapted to implement the method.
2. Description of the Prior Art
U.S. Pat. No. 6,219,417 describes an analog telephone line equipment including a hybrid resistor array and a signal processor that is also used to carry out tests, in particular when sending a ringing signal over the telephone line. However, it does not describe any test for checking the integrity of the hybrid resistor array.
The document EP 0 808 053 describes an analog telephone line equipment including a hybrid resistor array and a signal processor that is also used to carry out tests. It describes how to isolate the equipment from the telephone line while testing the equipment without using relays and a test for checking the integrity of the hybrid resistor array. The signal processor supplies a 1 kHz digital test signal to a codec which converts the signal into an analog signal which is injected into the hybrid resistor array. The signal reflected by the hybrid array is amplified by the telephone signal receiver amplifier, after which its amplitude is measured by the signal processor. Its amplitude depends on the load on each of the wires of the telephone line and on the state of the hybrid resistor array. If the array includes a resistor that is open circuit, the reflected signal has an amplitude above a fixed threshold, and the processor trips an alarm. The above test method has the drawback that it can be applied only to an equipment including a signal processor. However, a signal processor considerably increases the cost of a telephone line equipment.
FIG. 1 shows a conventional line equipment designed to be connected by a telephone line 101 to a basic telephone, i.e. a telephone that operates over an ordinary analog line, as opposed to new systems such as an ISDN or ADSL, for example. The lines and the units adapted to send and receive voice signals between the equipment and the local office are known in the art and are not shown, to simplify the figure and because they are not relevant to the invention.
The telephone line is connected at the input of the line equipment to an overvoltage protection device 102 of a type that is known in the art. The device 102 is connected to a line test relay 103 that is used to isolate the line equipment from the telephone line in order to connect it to a line test bus 104.
A second equipment test relay 105 is used to test the operation of the line equipment, in particular the on-hook/off-hook function, and more particularly the integrity of the hybrid resistor array 107, by isolating the input of the array 107 in order to connect it to an equipment test bus 106. Thus outside these test periods the relay 105 connects the line from the relay 103 to the input of the hybrid array 107.
The hybrid array 107, the detailed structure of which is described later, is connected through a ringing relay 108 either to a power supply circuit 109 or to a ringing circuit 111. In the present example, the circuit 109 is connected to two switching circuits of the relay and corresponds, for one of them, to the negative side of the power supply (Vbat) and, for the other of them, to the positive side of the power supply. However, as is known in the art, the negative side is the hot side and the positive side is connected to the general ground 110.
The control circuits for these various relays are known in the art and are therefore not shown in the figure.
The output of the hybrid array 107 is connected to a circuit 12 for detecting the off-hook or on-hook state of the telephone connected to the line equipment. A voltage Vcc applied to the hybrid array 107 via a connection 114 provides a bias voltage that is used by the circuit 112. If the on-hook state is detected, the circuit 112 sends a logic signal to an output connection 113. It sends a complementary logic signal if it detects the on-hook state.
Using a test relay 105 to test the hybrid array 107 generates a cost that is all the higher in that there are as many relays as there are line equipments and therefore telephones served by the system.